The present invention generally relates to the production of semiconductor devices and more particularly to a charged particle beam exposure apparatus used in producing semiconductor devices.
Electron beam lithography is an essential technology in the production of advanced semiconductor integrated circuits having high integration. By using electron beam lithography, it is possible to expose the resist with a resolution of less than 0.05 .mu.m and an alignment error of less than 0.02 .mu.m. It is thought that electron beam lithography promises to play a central role in the future production of semiconductor devices such as a DRAM having a storage capacity exceeding 256 megabits or a high-speed microprocessor equipped with a significantly powerful operational capability.
A charged particle beam exposure apparatus generally comprises: a charged particle beam source for generating and emitting a charged particle beam, and for directing the beam along a predetermined optical axis to an article to be exposed; an electron lens which, provided around the optical axis, focuses the charged particle beam on the article; a deflector which deflects the charged particle beam with respect to the optical axis and which scans the surface of the article with the charged particle beam; a movable stage which, provided along the optical axis, movably holds the article. A desired semiconductor pattern is produced on the article by the charged particle.
FIG. 1 shows the schematic constitution of a conventional charged particle beam exposure apparatus or an electron beam exposure apparatus.
Referring to FIG. 1, an electron beam exposure apparatus includes a vacuum system 1 composed of a vacuum chamber 1a from which the air inside is evacuated through a valve 2a by means of a vacuum pump 2 and a vacuum column 1b extending above the vacuum chamber 1a. A movable stage 3 for supporting a semiconductor substrate 4 is provided in the vacuum chamber 1a. The movable stage 3 comprises a first stage plate 3a supported by a roller 3c so as to be movable in a X direction and a second stage plate 3b supported by a roller 3d so as to be movable in a Y direction with respect to the first stage plate. The vacuum chamber 1a is also provided with a driving mechanism (not shown) for moving the first and second stage plates 3a and 3b in the X and Y directions, respectively.
The vacuum column 1b is provided with an electron gun 5 for forming an electron beam along an optical axis O. The electron beam thus formed travels along the optical axis O to hit the substrate 4 so as to expose an electron beam resist formed on the substrate 4. An electron lens 6 for focusing the electron beam on the surface of the substrate 4 is provided around the optical axis O in the vacuum column 1b. An electron beam path 6a is formed around the optical axis O by the electron lens 6. A deflector 7 for deflecting the electron beam is formed in a part of the path 6a. By driving the deflector 7, the surface of the substrate 4 is scanned by the electron beam. By driving the deflector 7 and the movable stage 3 in a coordinated manner, it is possible to scan most of the surface of the substrate 4. A reflected electron detector 10 is provided in the lower end of the electron lens 6 opposite the substrate 4 so as to detect reflected electrons emitted by the substrate in response to the irradiation of the substrate 4 by the electron beam. The vacuum chamber 1a is also provided with a secondary electron detector 9 for detecting low-energy energy secondary electrons emitted by the substrate 4 in response to the irradiation thereof. These detectors are used in order to produce a substrate image characteristic of a scanning electron microscope.
A vacuum sub-chamber 1c is provided so as to communicate with the vacuum chamber 1a via a gate valve 1d. An entrance gate valve 1e is provided in the vacuum sub-chamber 1c so that the substrate 4 can be loaded into and removed from the vacuum chamber 1a. An arm mechanism 1f for introducing the substrate 4 into the vacuum sub-chamber 1c and transporting the same onto the stage 3 and a driving mechanism 1g for driving the arm mechanism 1f are also provided in the vacuum sub-chamber 1c.
Semiconductor device production yield is greatly reduced when dust collects on a substrate. Therefore, much effort has been put into eliminating dust as much as possible. The effect of dust is particularly serious in the case of semiconductor devices having a fine pattern produced by an electron beam exposure apparatus. As many as several thousands to several tens of thousands of dust particles are detected when a measurement is made of dust remaining on the substrate removed from the vacuum chamber after it has been introduced into the vacuum chamber in a clean state and then exposed. Accordingly, measures have been taken in the electron beam exposure apparatus as shown in FIG. 1 by which sliding portions such as rollers or bearings have been covered so as to prevent dust from being dispersed. Other measures taken are such that, when the substrate is removed from the vacuum system to an environment under atmospheric pressure, or when the substrate is introduced from the environment under atmospheric pressure into the vacuum chamber, suction of air into the vacuum system as well as evacuation therefrom is performed in a significantly slow manner so as to prevent dust from being dispersed. However, these measures are not effective enough to reduce by a satisfactory amount dust adhering to the substrate after it is exposed. For example, even if the sliding portions of the vacuum system are covered, it is inevitable that as many as several hundreds to several thousands of dust particles will remain on a 6-inch wafer.
The following facts were discovered by the inventors of the present invention while experimenting with the production of a semiconductor pattern on the substrate 4 by using the electron beam exposure apparatus as shown in FIG. 1.
1) A clean 6-inch substrate 4 was introduced into the vacuum sub-chamber 1c, which was then evacuated, and then air was sucked thereinto. When the substrate 4 was removed from the vacuum system to an environment under atmospheric pressure and the amount of dust was measured, it was found that there was little dust adhering to the substrate.
2) The substrate was introduced into the vacuum sub-chamber 1c, which was then evacuated. The arm 1f was then driven so as to move the substrate onto the stage 3, and then the substrate was moved back to the vacuum sub-chamber 1c unprocessed. A subsequent measurement revealed that there was little dust adhering to the substrate 4.
3) The process of driving the stage 3 so as to move the substrate 4 was added to the experimental process described in 2). It was found subsequently that there was still little dust adhering to the substrate 4.
4) The process of applying the electron beam on the substrate without moving the stage was added to the experimental process described in 2). It was found subsequently that there was still little dust adhering to the substrate 4.
5) The processes of 3) and 4) were carried out at the same time. A subsequent measurement revealed as many as several hundreds to several thousands of dust particles adhering to the substrate.
Since the process of 5) is indispensable when using the electron beam exposure apparatus, at least several hundred dust particles adheres to the substrate. Although the actual amount of dust depends on the exposure time, a reduction in the semiconductor device production yield is inevitable. The fact that a large amount of dust adheres to the substrate only when the process of 5) is carried out provides a useful hint as to how dust adheres to the surface of a substrate. That is, the dust adhering to the substrate after the exposure process is thought to be generated by the moving of the sliding parts of the vacuum system and charged in response to the application of the electron beam on the substrate. It is supposed that the entity which causes the dust to be charged is not the focused electron beam itself but reflected electrons, low-energy secondary electrons e.sup.- or positive ions generated by applying the electron beam on the substrate. These low-energy charged particles are generated in a large amount in the vacuum chamber 1a. These particles fill the vacuum chamber 1a and negatively charge dust d.sup.- stirred up by the moving mechanism of the movable stage 3. The charged dust d.sup.- thus generated collects on the surface of the substrate 4 to cause a defect in the substrate.